Power semiconductor devices are widely used to carry large currents, support high voltages and/or operate at high frequencies such as radio frequencies. A wide variety of power semiconductor devices are known in the art including, for example, power switching devices and power amplifiers. Many power semiconductor devices are implemented using various types of Field Effect Transistors including, for example, High Electron Mobility Transistors (HEMT) and Metal Oxide Semiconductor Field Effect Transistors (MOSFETs).
Modern power semiconductor devices are generally fabricated from wide bandgap semiconductor materials. For example, power HEMTs may be fabricated from gallium arsenide (GaAs) based material systems or, more recently, from gallium nitride (GaN) based material systems that are formed on a silicon carbide (SiC) substrate. Power semiconductor devices may be formed as discrete devices or as a plurality of devices (which may include transistors and other circuit devices such as resistors, inductors, capacitors, transmission lines and the like) that are formed on a common substrate to provide a so-called Monolithic Microwave Integrated Circuit (MMIC). A MMIC refers to an integrated circuit that operates on radio and/or microwave frequency signals in which all of the circuitry for a particular function is integrated into a single semiconductor chip. An example MMIC device is a transistor amplifier that includes associated matching circuits, feed networks and the like that are all implemented on a common substrate. MMIC transistor amplifiers typically include a plurality of unit cell HEMT transistors that are connected in parallel.
Field effect transistors such as HEMTs and MOSFETs may be classified into depletion mode and enhancement mode types, corresponding to whether the transistor is in an ON-state or an OFF-state at a gate-source voltage of zero. In enhancement mode devices, the devices are OFF at zero gate-source voltage, whereas in depletion mode devices, the device is ON at zero gate-source voltage. HEMTs are typically implemented as depletion mode devices, in that they are conductive at a gate-source bias of zero due to the polarization-induced charge at the interface of the barrier and channel layers of the device.